Bilingual edition: English is preserved and Chinese follows each unit. Terminology uses the confirmed v20260916 glossary; automated semantic review remains traceable.
中英双语版：英文原文完整保留，中文紧随对应单元；术语采用 v20260916 确认表，自动语义审校结果可追溯。

Relative Motion

相对运动

The laws of physics which apply when you are at rest on the earth also apply when you are in any reference frame which is moving at a constant velocity with respect to the earth. For example, you can toss and catch a ball in a moving bus if the motion is in a straight line at constant speed.

物理学的定律在你静止在地球上时适用，也适用于任何以恒定速度相对于地球运动的参考系。例如，如果你在行驶的公交车内沿直线以恒定速度运动，你可以投掷并接住一个球。

The motion may have a different appearance as viewed from a different reference frame, but this can be explained by including the relative velocity of the reference frame in the description of the motion.

物体的运动在不同参考系中可能看起来不同，但这可以通过在运动描述中包含参考系的相对速度来解释。
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Relative Velocity

相对速度

One must take into account relative velocities to describe the motion of an airplane in the wind or a boat in a current. Assessing velocities involves vector addition and a useful approach to such relative velocity problems is to think of one reference frame as an "intermediate" reference frame in the form:

为描述飞机在风中或船在水流中的运动，必须考虑相对速度。评估速度涉及矢量相加，解决此类相对速度问题的一个有用方法是将一个参考系视为“中间”参考系，形式为：

Put into words, the velocity of A with respect to C is equal to the velocity of A with respect to B plus the velocity of B with respect to C. Reference frame B is the intermediate reference frame. This approach can be used with the airplane or boat examples.

用语言表达，A相对于C的速度等于A相对于B的速度加上B相对于C的速度。参考系B是中间参考系。这种方法可以用于飞机或船的例子。
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HyperPhysics***** Mechanics
力学
R Nave
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